Sending command through SPI on Arduino UNO

Question

I am assuming the default pin configuration is pin 10(SS), 11(MOSI), 12(MISO), 13(CLK) from a default setup, SPI.begin(). The code compiles OK. Not sure about the int valuex and using the SPI.transfer() function.

#include <SPI.h>
int value1 = 0x8026;
int value2 = 0x0801;
int value3 = 0x8027;
int value4 = 0x0000;
void setup (void)
{
 Serial.begin (9600);
 Serial.println ("Starting");
 digitalWrite(SS, HIGH); // ensure SS stays high for now
 // Put SCK, MOSI, SS pins into output mode
 // also put SCK, MOSI into LOW state, and SS into HIGH state.
 // Then put SPI hardware into Master mode and turn SPI on
 SPI.begin ();
 // Slow down the master a bit
 SPI.setClockDivider(SPI_CLOCK_DIV4);
} // end of setup
void loop (void)
{
 // enable Slave Select
 digitalWrite(SS, LOW); 
 SPI.transfer (1); // initiate transmission
 SPI.transfer (value1);
 SPI.transfer (value2);
 SPI.transfer (value3);
 SPI.transfer (value4);
 // disable Slave Select
 digitalWrite(SS, HIGH);
delay (1000); 
}

• DataFiddler: transfer sends a byte only. See SPI.transfer16(intval); if necessary

Answer 1

Not sure about the int valuex and using the SPI.transfer() function.

A couple of things to know:

1. You are storing an unsigned 16-bit literal into a signed 16-bit variable. You should be using uint16_t value1 = 0x8026; etc. Also, unless you are planning to change the values, they should be const.
2. SPI.transfer() transfers only 8 bits of data. You are only sending the lowest byte of your valuex variables. SPI.transfer16() sends an unsigned 16-bit value.

To illustrate, this is how your program should look:

#include <SPI.h>
const uint16_t value1 = 0x8026;
const uint16_t value2 = 0x0801;
const uint16_t value3 = 0x8027;
const uint16_t value4 = 0x0000;
void setup(void) {
 Serial.begin(9600);
 Serial.println("Starting");
 digitalWrite(SS, HIGH); // ensure SS stays high for now
 // Put SCK, MOSI, SS pins into output mode
 // also put SCK, MOSI into LOW state, and SS into HIGH state.
 // Then put SPI hardware into Master mode and turn SPI on
 SPI.begin();
 // Slow down the master a bit
 SPI.setClockDivider(SPI_CLOCK_DIV4);
} // end of setup
void loop(void) {
 // enable Slave Select
 digitalWrite(SS, LOW);
 SPI.transfer(1); // initiate transmission
 SPI.transfer16(value1);
 SPI.transfer16(value2);
 SPI.transfer16(value3);
 SPI.transfer16(value4);
 // disable Slave Select
 digitalWrite(SS, HIGH);
 Delay(1000);
}

That will transfer the stream of 8-bit values:

0x01 0x80 0x26 0x08 0x01 0x80 0x27 0x00 0x00

• howdyrichard: Hi, and thank you. I believe the code is right but I do not get any signaling out of MOSI pin 11; it does go from high to low. I can 'see' (with an oscilloscope) SS pin 10 go low, I can see only 3 clock pulses on pin 13. I am not concerned with any incoming data.
• howdyrichard: Also, when I added 16 to SPI.transfer, transfer changed from blue to black color.
• howdyrichard: The code works perfect! Thank you and I wish I had sent this sooner. Thanks again!
• SoreDakeNoKoto: @howdyrichard Accept the answer if it worked for you